The present invention relates to a pump displacement control system for a hydraulic drive apparatus provided with a load sensing system for controlling the displacement of a hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a set differential pressure. More particularly, the present invention relates to a pump displacement control system for controlling the displacement of a hydraulic pump in link with an engine revolution speed, and a valve unit for use in the pump displacement control system.
As one hydraulic system for controlling actuators of a hydraulic excavator, there is known the so-called load sensing system including a pump displacement control system wherein respective load pressures of the actuators are detected and the delivery rate of a hydraulic pump is controlled so that the delivery pressure of the hydraulic pump is provided by a pressure equal to the sum of a maximum one of the detected load pressures and a certain set differential pressure. The set differential pressure in such a load sensing system (hereinafter referred to also as the LS set differential pressure) is usually set to a certain constant value (e.g., 15 bar) by biasing means such as a spring.
Also, JP-U-2-149881 and JP-A-5-99126 each disclose a pump displacement control system which enables an actuator speed to be changed in link with an engine revolution speed in the above-described ordinary load sensing system.
In the pump displacement control system disclosed in JP-U-2-149881, a throttle is disposed in a delivery line of a fixed displacement pump that is provided as a hydraulic source of a pilot hydraulic circuit for operating equipment such as a group of hydraulic remote control valves. A pressure upstream of the throttle is detected as a signal pressure Pc, and the detected signal pressure Pc is introduced via a signal hydraulic line to a pressure bearing sector of a load sensing valve on the same side as a pressure bearing sector to which a load pressure Pls is introduced. Since the pressure upstream of the throttle changes depending on the revolution speed of the fixed displacement pump, this means that the detected signal pressure Pc contains information of the revolution speed.
The pump displacement control system disclosed in JP-A5-99126 comprises a servo piston for tilting a swash plate of a variable displacement hydraulic pump, and a tilting control unit for performing displacement control such that, depending on a differential pressure xcex94PLS between a delivery pressure Ps of the hydraulic pump and a load pressure PLS of an actuator driven by the hydraulic pump, a pump delivery pressure is supplied to the servo piston so as to maintain the differential pressure xcex94PLS at a set value xcex94PLSref. The disclosed pump displacement control system further comprises a fixed displacement hydraulic pump driven by an engine together with the variable displacement hydraulic pump, a throttle provided in a delivery line of the fixed displacement hydraulic pump, and means for varying the set value xcex94PLSref of the tilting control unit depending on a differential pressure xcex94Pp across the throttle. The engine revolution speed is detected in accordance with change of the differential pressure across the throttle provided in the delivery line of the fixed displacement hydraulic pump, and the set value xcex94PLSref of the tilting control unit is varied depending on the detected engine revolution speed.
In a hydraulic drive apparatus provided with a typical conventional load sensing system wherein the set differential pressure of a load sensing valve is given by a spring, even when the engine revolution speed is lowered, the displacement of a hydraulic pump is not changed and the flow rate of a hydraulic fluid supplied to an actuator is also not changed. Accordingly, the actuator speed cannot be slowed down in link with the engine revolution speed. The working speed can be regulated by adjusting the throttle opening of a flow control valve, but to this end a control lever for adjusting the throttle opening of the flow control valve must be operated while holding a lever position within an intermediate stroke range. To improve fine operability, it is desired that, even with the control lever held at a full stroke position, when the engine revolution speed is lowered, the maximum actuator speed (maximum flow rate of the hydraulic fluid supplied to the actuator) can be reduced correspondingly for adjustment of the maximum working speed.
In the pump displacement control system disclosed in JP-U-2-149881, the set differential pressure of the load sensing valve is given by the signal pressure Pc that is obtained by detecting the pressure upstream of the throttle provided in the delivery line of the fixed pump. As a result, with a decrease of the engine revolution speed, the signal pressure (pressure upstream of the throttle) Pc is lowered, which in turn lowers the set differential pressure of the load sensing valve, whereby the displacement of the hydraulic pump is reduced and the working speed of the actuator is slowed down. It is hence possible to control the displacement of the hydraulic pump and adjust the working speed in link with the engine revolution speed.
In the disclosed pump displacement control system, the pilot hydraulic circuit is provided to produce a signal pressure for operating the equipment such as a group of hydraulic remote control valves, and the pressure downstream of the throttle for detecting the engine revolution speed is set by a relief valve for setting a primary pilot pressure. Letting Pa be the pressure set by the relief valve and Pb be the pressure loss caused by the throttle for detecting the engine revolution speed, the pressure (signal pressure) Pc upstream of the throttle is expressed by Pc=Pa+Pb.
Assuming, for example, that the set pressure Pa of the relief valve for setting the primary pilot pressure is 45 bar, the delivery rate of the fixed pump at the engine revolution speed of 2000 rpm is 35 liter/min (the set pressure Pa is assumed to be kept at 45 bar even upon consumption of the pump delivery rate), and the pressure loss Pb caused by the throttle for detecting the engine revolution speed is 15 bar, the pressure Pc upstream of the throttle is 60 bar. In the typical conventional load sensing system wherein the set differential pressure of the load sensing valve is given by a spring, an equivalent pressure applied by the spring is, e.g., about 15 bar. To provide the set differential pressure at a value equal to 15 bar in the pump displacement control system disclosed in JP-U-2-149881, the pressure bearing sector of the load sensing valve is required to modulate 60 bar of the throttle upstream pressure Pc down about xc2xc, i.e., to 15 bar. Providing such a function to modulate the pressure results in a more complicated structure of the load sensing valve.
In the pump displacement control system disclosed in JP-A-5-99126, the set value xcex94PLSref of the tilting control unit is varied depending on the differential pressure xcex94Pp across the throttle instead of the pressure Pc upstream of the throttle for detecting the engine revolution speed. The differential pressure xcex94Pp across the throttle coincides with the pressure loss Pb caused in the throttle, and is 15 bar in the above-mentioned example. This value is equal to the equivalent pressure applied by the spring, i.e., about 15 bar, which is provided in the typical conventional load sensing system. Accordingly, when the differential pressure xcex94Pp across the throttle is employed instead of the pressure Pc upstream of the throttle, the differential pressure xcex94Pp across the throttle can be directly introduced to act upon the pressure bearing sector of the load sensing valve and the structure of the load sensing valve can be avoided from being complicated. This prior art, however, has a problem as follows.
When the rated revolution speed of the engine is 2000 rpm as mentioned above and the idling revolution speed of the engine is 1000 rpm, the engine revolution speed varies over the range of 1000-2000 rpm. On the other hand, assuming that the differential pressure across the throttle for detecting the engine revolution speed is 15 bar as mentioned above when the engine revolution speed is 2000 rpm, the differential pressure across the throttle developed when the engine revolution speed is 1000 rpm is 7.5 bar. Hence, the differential pressure across the throttle is changed over the range of 7.5-15 bar while the engine revolution speed varies over the range of 1000-2000 rpm. This means that the set differential pressure is changed over the range of 7.5-15 bar for the variable range of 1000-2000 rpm of the engine revolution speed, and that the set differential pressure cannot be reduced down to a level below 7.5 bar. It has been therefore impossible to reduce the displacement of the hydraulic pump down beyond a certain value in the idling revolution range where the work amount is relatively small, to overcome a limitation in improvement of fine operability, and to cut down fuel consumption.
An object of the present invention is to provide a pump displacement control system which enables a pressure varying in link with an engine revolution speed to be directly employed as the set differential pressure of a load sensing valve, thereby avoiding the structure of the load sensing valve from being complicated, and which can reduce the displacement of a hydraulic pump down in the idling revolution range where the work amount is relatively small, thereby improving fine operability and cutting down fuel consumption, as well as a valve unit for use in the pump displacement control system.
(1) To achieve the above object, the present invention provides a pump displacement control system provided in a hydraulic drive apparatus comprising an engine and a variable displacement hydraulic pump driven by the engine for rotation and supplying a hydraulic fluid to a plurality of actuators through respective flow control valves, the pump displacement control system comprising a load sensing valve for controlling a displacement of the hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among the plurality of actuators is maintained at a target differential pressure, a fixed displacement hydraulic pump driven by the engine for rotation together with the variable displacement hydraulic pump, and a throttle provided in a delivery line of the fixed displacement hydraulic pump, the displacement of the variable displacement hydraulic pump being controlled by detecting change of a revolution speed of the engine and modifying the target differential pressure in accordance with change of a differential pressure across the throttle, wherein the pump displacement control system further comprises differential pressure detecting means for detecting the differential pressure across the throttle and outputting, as a signal pressure, a pressure lower than the detected differential pressure by a predetermined value whereby the target differential pressure of the load sensing valve is set based on the outputted signal pressure.
By thus providing the differential pressure detecting means which outputs, as the signal pressure, the pressure lower than the differential pressure across the throttle by the predetermined value, and setting the target differential pressure of the load sensing valve based on the outputted signal pressure, the above-mentioned problems are solved as follows.
1) Since the pressure (signal pressure) lower than the differential pressure across the throttle by the predetermined value, i.e., the output of the differential pressure detecting means, contains information of the engine revolution speed, the displacement of the hydraulic pump can be controlled in link with the engine revolution speed. Further, since the differential pressure across the throttle rather than the pressure upstream of the same is detected as the signal pressure in link with the engine revolution speed, the signal pressure can be employed on the side of the load sensing valve to set the target differential pressure, and the structure of the load sensing valve can be simplified.
2) By setting an opening area of the throttle such that, at the rated revolution speed of the engine, the pressure lower than the differential pressure across the throttle by the predetermined value, i.e., the output of the differential pressure detecting means, is equal to the differential pressure across a throttle in a conventional system wherein the differential pressure across the throttle is employed as it is, a decrease rate of the differential pressure across the throttle with respect to the engine revolution speed is greater than that in the conventional system. Therefore, the output of the differential pressure detecting means in the idling revolution range becomes smaller than the differential pressure across the throttle in the conventional system. As a result, in the idling revolution range in which the work amount is relatively small, the displacement of the hydraulic pump can be reduced to improve fine operability and cut down fuel consumption.
(2) In above (1), preferably, the differential pressure detecting means is a differential pressure detecting valve including a first pressure bearing section to which a pressure upstream of the throttle is introduced and which acts to connect the output side of the differential pressure detecting valve itself to the upstream side of the throttle, a second pressure bearing section to which a pressure downstream of the throttle is introduced and which acts to connect the output side of the differential pressure detecting valve itself to a reservoir, a third pressure bearing section to which a pressure on the output side of the differential pressure detecting valve itself is introduced and which acts to connect the output side of the differential pressure detecting valve itself to the reservoir, and a spring acting to connect the output side of the differential pressure detecting valve itself to the reservoir and setting the predetermined value.
With those features, the differential pressure detecting means operates to lower the output thereof from the differential pressure across the throttle by the predetermined value that is provided as a set value of the spring, thereby outputting the pressure lower than the differential pressure across the throttle by the predetermined value.
(3) In above (1), preferably, the differential pressure detecting means is constituted as an integral valve unit together with the throttle, the valve unit comprising a pump port connected to a delivery line of the fixed displacement hydraulic pump, a reservoir port connected to the reservoir, a circuit port connected to a pilot hydraulic circuit operating by a hydraulic fluid delivered from the fixed displacement hydraulic pump, and a load sensing port connected to the load sensing valve; a spool formed therein with a throttle passage for communicating the pump port and the circuit port with each other at all times and functioning as the throttle, a first notch for controlling communication between the pump port and the load sensing port, and a second notch for controlling communication between the load sensing port and the reservoir port; and spool biasing means for selectively opening the first notch and the second notch to produce, in the load sensing port, the pressure lower than the differential pressure across the throttle by the predetermined value.
By thus constituting the differential pressure detecting means as the integral valve unit together with the throttle, an integrated unit of the throttle and the pressure detecting means can be realized with a simplified construction.
(4) In above (3), preferably, the throttle passage formed in the spool has a throttle hole being open in the radial direction of the spool.
With that feature, since no fluid forces are caused in the throttle passage, an effect of fluid forces upon the spool stroke can be eliminated and a precise signal pressure in link with the engine revolution speed can be produced.
(5) In above (3), preferably, the spool biasing means comprises a first pressure bearing section to which a pressure in the pump port is introduced and which is formed to bias the spool in the opening direction of the first notch, a second pressure bearing section to which a pressure in the circuit port is introduced and which is formed to bias the spool in the opening direction of the second notch, a third pressure bearing section to which a pressure in the load sensing port is introduced and which is formed to bias the spool in the opening direction of the second notch, and a spring acting upon the spool to bias the spool in the opening direction of the second notch for thereby setting the predetermined value.
With those features, the spool biasing means selectively opens the first notch and the second notch to produce, in the load sensing port, the pressure lower than the differential pressure across the throttle by the predetermined value.
(6) Further, to achieve the above object, the present invention provides a valve unit which is provided in a delivery line of a fixed displacement hydraulic pump driven by an engine for rotation together with a variable displacement hydraulic pump, outputs a signal pressure depending on a revolution speed of the engine, and sets a target differential pressure of a load sensing valve associated with the variable displacement hydraulic pump, wherein the valve unit comprises a pump port connected to a delivery line of the fixed displacement hydraulic pump, a reservoir port connected to the reservoir, a circuit port connected to a pilot hydraulic circuit operating by a hydraulic fluid delivered from the fixed displacement hydraulic pump, and a load sensing port for outputting the signal pressure; a spool formed therein with a throttle passage for communicating the pump port and the circuit port with each other at all times and functioning as the throttle, a first notch for controlling communication between the pump port and the load sensing port, and a second notch for controlling communication between the load sensing port and the reservoir port; and spool biasing means for selectively opening the first notch and the second notch to produce, in the load sensing port, a pressure lower than a differential pressure across the throttle by a predetermined value.
The target differential pressure of the load sensing valve is thus set by producing the pressure lower than the differential pressure across the throttle by the predetermined value, and outputting the produced signal pressure as the signal pressure. By so setting the target differential pressure, as described in the foregoing 1) and 2), the structure of the load sensing valve can be avoided from being complicated, and in the idling revolution range in which the work amount is relatively small, the displacement of the hydraulic pump can be reduced to improve fine operability and cut down fuel consumption.
Furthermore, as described in the foregoing (3), an integrated unit of the throttle and the pressure detecting means can be realized with a simplified construction.
(7) In above (6), preferably, the throttle passage formed in the spool has a throttle hole being open in the radial direction of the spool.
With that feature, similarly to the foregoing (4), an effect of fluid forces otherwise caused in the throttle passage can be eliminated and a precise signal pressure in link with the engine revolution speed can be produced.
(8) In above (6), preferably, the spool biasing means comprises a first pressure bearing section to which a pressure in the pump port is introduced and which is formed to bias the spool in the opening direction of the first notch, a second pressure bearing section to which a pressure in the circuit port is introduced and which is formed to bias the spool in the opening direction of the second notch, a third pressure bearing section to which a pressure in the load sensing port is introduced and which is formed to bias the spool in the opening direction of the second notch, and a spring acting upon the spool to bias the spool in the opening direction of the second notch for thereby setting the predetermined value.
With those features, similarly to the foregoing (5), the spool biasing means selectively opens the first notch and the second notch to produce, in the load sensing port, the pressure lower than the differential pressure across the throttle by the predetermined value.